The instant invention relates to methods of and systems for oscillation damping of trajectories and stops for simply suspended payloads by utilizing controlled acceleration.
In material handling applications, the use of intelligent computer-controlled machinery, such as computer-controlled bridge cranes, is now under very serious consideration. A particular application for this technology is the handling of material at nuclear waste storage and dump sites where 20-foot-long casks containing nuclear waste materials weighing up to 100 tons must be moved to various locations throughout the sites. It is imprudent to have personnel standing in a nuclear waste dump to assist in positioning payloads being loaded into the dump not only because of radiation hazards but also because of the size and weight of the casks. Accordingly, it is necessary to lower these articles with precision while being controlled from above. When payloads are relatively heavy and are transported by bridge cranes into position, the payloads conventionally continue to swing back and forth after the trolley of the crane has stopped moving. This happens for even minor adjustments in the position of the payload. Consequently, the task of lowering or positioning a storage cask in a nuclear waste dump or storage site could be quite protracted and aggravating when relying on conventional approaches where the bridge crane operator relies on his skill to damp the motion of a payload.
It has been found that damping oscillations of payloads simply suspended from cranes is possible if the acceleration of the crane or manipulator is programmable. Additionally, residual oscillation remaining after transportation of the payload is complete may be damped by servoing off the forces created by the pendulum motion if the transporting device can be operated under computer control.
While the prior art as exemplified by the patent literature has considered these problems, there are drawbacks in the prior art of which the inventors are aware. U.S. Pat. No. 4,603,783 is directed to cosine acceleration and discloses a method of taking full advantage of the torque capability of an available motor to accomplish a swing-damped trajectory and swing-free stop. In this patent, the payload under consideration may weigh as much or more than that of the bridge/trolley. Consequently, the crane trolley acceleration is maximum when the payload is directly under the trolley and is reduced as the payload swings out. However, the cube-root formula describing the pendulum period appears incorrect and the small angular formula is used for the period of oscillation. U.S. Pat. No. 4,756,432 is directed to a two-pulse method in which mid-course constant velocity is somehow controlled. However, there appear to be some deficiencies in this disclosure. For example, it is not understood why one would decrease velocity during mid-course transport, and there is no indication that the friction working against the load during acceleration and helping during deceleration is accounted for. There is a discussion of rope length and load swing However, the period of swing is of great importance, and it is stated that running resistance is a function of "x" when it should be independent of position. Finally, it would appear that constant velocity during acceleration and deceleration would be desirable because it decreases the overall time of a move, rather than allowing a reduction in velocity between pulses when accelerating and an increase in velocity between pulses during deceleration.
U.S. Pat. No. 4,717,029 discloses full period acceleration. However, rope length is used to determine the period instead of the length to the center of gravity of the pendulum. Moreover, in this patent the change in period due to crane acceleration is neglected.
Considering older patents, U.S. Pat. No. 3,517,830 utilizes two pulses or a one-period pulse, but does not account for the period change due to acceleration U.S Pat. No. 3,921,818 directed to two-pulse acceleration measures tension .theta. and .theta. which would allow one to do a considerable amount of tuning or to develop special profiles. However, in this patent, coasting is allowed between acceleration and deceleration pulses. When one considers the effect of friction, the coasting really creates confusion by providing different and possibly unpredictable velocity changes during the acceleration and deceleration phases of a move.
In view of the deficiencies of the prior art in addressing the problems of providing a method of and apparatus for swing-damping movement of suspended objects, there is a need for more practical viable approaches.